Part 2: Core Practicals 13a and 13b: obtaining a rate law

Follow iodine concentration by timed titration, then use a clock threshold to compare initial rates while controlling the mixture composition.

CP13a: why iodine gives a useful concentration signal

Acid-catalysed iodination of propanone follows the overall equation CH₃COCH₃ + I₂ → CH₃COCH₂I + H⁺ + I⁻. Iodine is consumed and can be measured by reaction with standard thiosulfate. Use propanone and acid in large excess so their concentrations change only slightly while iodine decreases.

The titration reaction is I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. One mole of iodine consumes two moles of thiosulfate. Starch is an indicator, not a catalyst; it forms a strongly coloured complex when iodine remains. Add it near the pale iodine end point, then finish to disappearance of the blue colour.

Because the aliquot volume and titrant concentration are fixed, titre is proportional to iodine concentration. An approximately straight downward titre–time graph therefore implies a constant rate of iodine consumption under those controlled conditions, showing zero order with respect to iodine.

Sample, quench and time the same chemical event

Prepare the prescribed dilute reaction mixture, start a continuously running timer when iodine is mixed in, and withdraw equal measured aliquots at planned intervals. Quench each promptly with sodium hydrogencarbonate, which removes the acid catalyst by neutralisation. Record the actual time of quenching, not merely the planned sample time.

Titrate each quenched aliquot with standard thiosulfate and record initial and final burette readings, titre and the corresponding time. For a normal burette scale, titre is final minus initial reading. Repeat the whole run if needed to assess variation; multiple titres of different kinetic times are not replicates of one concentration.

A slow or inconsistent transfer/quench leaves time for extra iodine consumption and associates too low an iodine concentration with the nominal earlier time. Prepare receiving flasks in advance and keep the transfer procedure consistent. Large aliquots progressively remove material from the vessel; sampling must leave enough well-mixed solution for the planned measurements.

The product iodopropanone strongly irritates eyes; use the teacher’s fume-handling and disposal arrangements and do not allow the reaction to continue beyond the prescribed short practical period. Propanone is flammable, acid can irritate, and quenching can release gas: use eye protection, no ignition source and unsealed suitable receiving vessels. Follow the current supervised method rather than treating disposal in an old worksheet as universal guidance.

Worked analysis and separate concentration series

An original 10.00 cm³ quenched aliquot needs 8.00 cm³ of 0.0100 mol dm⁻³ thiosulfate. n(S₂O₃²⁻) = 0.00800 ×0.0100 = 8.00 ×10⁻⁵ mol, so n(I₂) = 4.00 ×10⁻⁵ mol. Original aliquot [I₂] = 4.00 ×10⁻⁵/0.01000 = 0.00400 mol dm⁻³. Dilution during quenching does not alter the iodine amount; use the original aliquot volume.

Suppose derived [I₂] values at 0, 60, 120 and 180 s are 0.00600, 0.00480, 0.00360 and 0.00240 mol dm⁻³. The constant slope is (0.00240 −0.00600)/180 = −2.00 ×10⁻⁵ mol dm⁻³ s⁻¹. Thus iodine disappears at 2.00 ×10⁻⁵ mol dm⁻³ s⁻¹, supporting zero iodine order over this range.

To find the other orders, use separate experiments varying the initial propanone concentration while acid and iodine concentrations stay fixed, then vary acid while propanone and iodine stay fixed. Keep total volume and temperature the same, using added water to compensate changed stock volumes. Use concentrations after mixing: cmixture = cstock ×Vstock/Vtotal.

A doubling of propanone concentration that doubles the initial slope magnitude indicates first order in propanone. An analogous doubling for [H⁺] indicates first order in H⁺. Along with zero iodine order, the observed law is rate = k[CH₃COCH₃][H⁺]. The single iodine-decay experiment alone cannot establish all three orders.

CP13b: the clock measures a fixed chemical threshold

In the Pearson peroxodisulfate–iodide clock, S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂ produces iodine. A small fixed amount of thiosulfate rapidly consumes it by I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. Once thiosulfate is exhausted, iodine accumulates and produces a blue-black starch colour.

The time is therefore the time to make a known small amount of iodine, not the time for the main reaction to finish. With fixed thiosulfate amount and fixed total volume, each trial reaches the same iodine concentration change. Average early rate = Δ[I₂]/t, so 1/t is proportional to initial rate if only a small fraction of reactants is consumed before the clock.

Keep thiosulfate and starch additions the same, vary only one main reactant at a time and compensate stock-solution volume with water. Allow all solutions to reach the same temperature, including freshly prepared starch. Use clean glassware because contamination can change catalysis, and start timing consistently on mixing. View the same end point against a white background.

The common name iodine clock covers several chemical systems, including hydrogen peroxide variants. Use the actual oxidant, equation and acid dependence of the mixture provided. Do not transfer the peroxodisulfate rate law automatically to another iodine clock.

Worked clock calculation and rate-law comparison

An original mixture of total volume 50.0 cm³ contains 5.00 cm³ of 0.0100 mol dm⁻³ thiosulfate. Its initial amount is 5.00 ×10⁻⁵ mol, so the clock requires 2.50 ×10⁻⁵ mol I₂, equivalent to Δ[I₂] = 5.00 ×10⁻⁴ mol dm⁻³. If t = 40.0 s, mean early iodine-production rate = 1.25 ×10⁻⁵ mol dm⁻³ s⁻¹.

If halving the mixed iodide concentration doubles the clock time from 40.0 to 80.0 s while other variables and threshold remain fixed, the rate halves: first order in iodide. If doubling peroxodisulfate concentration halves time to 20.0 s, that reactant is also first order. Hence rate = k[S₂O₈²⁻][I⁻], even though the overall equation consumes two iodide ions.

For initial [S₂O₈²⁻] = 0.0400 and [I⁻] = 0.0800 mol dm⁻³ in the 40.0 s trial, k ≈ 1.25 ×10⁻⁵/(0.0400 ×0.0800) = 3.91 ×10⁻³ dm³ mol⁻¹ s⁻¹. The value is approximate because the clock gives an average over an early finite interval, not an exact instantaneous rate.

Use calibrated volumetric apparatus when small stock volumes would have large percentage uncertainty. A ±0.4 s timing uncertainty is 1.0% of 40.0 s but 10% of 4.0 s. Very short times magnify mixing and reaction-time errors; suitably slower conditions and repeated runs are more useful than reporting extra timer digits.

Evaluate the assumption that makes each method valid

For CP13a, the key assumptions are an effective quench, accurate aliquot volume, selective iodine analysis and nearly constant excess propanone/acid concentrations. For CP13b, the key assumptions are a constant small clock threshold, controlled mixed concentrations and temperature, and a fast iodine–thiosulfate removal reaction. Explain which assumption an improvement protects.

Using too much thiosulfate delays the clock so far that the main reactants change appreciably: 1/t then describes a broader average rate, weakening the initial-rate approximation. Changing its amount between runs changes the threshold, making the reciprocal times incomparable even if concentrations were otherwise identical.

Wear eye protection and use the prescribed dilute solutions; peroxodisulfate is an oxidant and can require additional controls at higher concentrations. A full written evaluation supports practical understanding, but supervised performance and records are needed for the separate 9CH0/04 practical endorsement.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources